As is well known, mass filters (e.g. quadrupoles with round rods or hyperbolic rods) are used in many mass spectrometers to separate one ion species from another. In a quadrupole mass filter, opposing rod pairs are connected together. For selecting ions with one or more mass/charge (m/z) ratios of interest, an RF voltage, superimposed by a DC voltage, is applied between the two rod pairs. That is, the rods of the first opposing rod pair are connected together such that the rods have the same phase as each other, while the rods of the second opposing rod pair are connected together such that the rods have the same phase as each other but opposite to the phase on the first rod pair.
The m/z ratios of interest can be selected by adjusting the combination of DC voltage and RF voltage amplitude to appropriate values. A selected m/z range of interest is stable within the quadrupole and will be transmitted through it. All other ions will be unstable and many will hit one or more of the quadrupole rods. Some ions hitting a surface of the rods can stick to that surface. The tendency for a striking ion to stick to the surface may depend on its sample class (molecular structure), its incident angle, its kinetic energy, the surface temperature, the surface roughness, and the surface material amongst other factors. Ions that stick to the surface can thereby modify the work function of the material of the surface and can form insulating layers which are prone to charging effects.
In most applications, cleaning of the rods is hardly needed and indeed cleaning typically is not required at all or only required at intervals of years. That is typically the case for small molecule applications for example. However, under the conditions of some extreme applications, in certain proteomics cases for example, employing a narrow isolation range of precursor ions and when whole proteome digests are analysed with very high loads on nano-LC columns, a charging effect may be visible after several months and in the very worst case after several days of full time operation. This charging effect will lower the overall transmission of the device and lead to a general sensitivity loss for the experiment. In order to recover the sensitivity, the quadrupole mass filter must be physically cleaned, which results in instrument downtime and service costs.